In recent years, thermal transfer systems have been developed to obtain prints from pictures generated electronically by a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converged into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element (and sometimes a black element) is placed face-to-face with a dye receiving element. The two elements are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta, or yellow signal. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
A problem has existed with the use of dye-donor elements for thermal dye-transfer printing because a thin support is required in order to provide effective heat transfer. For example, when a thin polyester film is employed, it softens when heated during the printing operation and can stick to the thermal printing head. This causes intermittent rather than continuous transport across the thermal head. The dye transferred thus may not appear as a uniform area, but rather as a series of alternating light and dark bands (i.e., chatter marks). Another defect called "smiles", which are crescent shaped low density areas, is produced in the receiving element by stretch-induced folds in the dye-donor element. Another defect is produced in the receiving element when abraded or melted debris from the backing layer builds up on the thermal head and causes streaks parallel to the travel direction which may extend over the entire image area. In extreme cases, sufficient friction is often created to tear the dye-donor element during printing. It would be desirable to eliminate such problems in order to have a commercially acceptable system.
In an attempt to solve the foregoing problems, dye-donor or thermal transfer imaging sheets employed in the industry frequently make use of an anti-stick or slipping layer coated on the side distant the thermal transfer layer. Materials disclosed for anti-stick coatings include linear thermoplastic as well as crosslinked polymers, with additives of either inorganic or organic materials and/or particulate materials to enhance some aspects of performance.
EPO Publication No. 314,348 discloses a backcoat (i.e., anti-stick coating) composition for a thermal transfer dye-sheet which is representative of many such coatings used in the industry. The EPO Publication discloses an anti-stick coating containing an organic resin comprising at least one polyfunctional material have a plurality of pendant or terminal acrylic groups per molecule available for cross-linking, at least 10% by weight of the polyfunctional material having 4-8 such acrylic groups per molecule; at least one linear organic polymer soluble or partially soluble in the resin; and comprising 1-40% by weight of the resin/polymer mixture, a slip agent selected from derivatives of long chain carboxylic or phosphoric acids, long alkyl chain esters of phosphoric acids, and long alkyl chain acrylates; an antistatic agent soluble in the resin; and a solid particulate antiblocking agent less than 5 .mu.m in diameter.
In the EPO Publication, the linear polymer is employed to impart flexibility to the cross-linked resin as well as to adjust coating viscosity and improve adhesion of the cured film to the substrate. The separate slip system consists of salts of stearic and hydroxy stearic acids; for example, lithium soaps, and salts of polyvalent metals and stearic acids (such as zinc stearate). Thicknesses recommended with these materials are on the order of 1-5 .mu.m, preferably 1 .mu.m. This places constraints on the antiblocking particulate size distribution.
There are several major disadvantages incurred with the use of the system disclosed by the EPO Publication. First, the slip properties of the film are imparted by mobile additives separate from the main cross-linked resin, which show a tendency to solvate dyes in the opposing layer and subsequently contaminate the printhead with the dyes. Also, the slip agents employed are salts of metals such as lithium or zinc, which are generally undesirable since these metals' ions can migrate into the opposing dye layers which may affect image properties, and into the head construction which may cause printing element failure. Additionally, talc, when used in sufficient quantities to be the main antiblocking agent, may require the thermal printer to use excessive pressure to ensure contact with the print head. This pressure can cause the inorganic particulate to abrade the printhead with time. Further, the average particle size of talc is reported as 2 .mu.m, which can be expected to result in distributed protrusions from the antistick coating. Finally, the recommended coating thickness of from 1 to 5 .mu.m contributes considerable thermal mass to the donor ribbon as a whole. With the standard 6 .mu.m PET carrier film, the ribbon can be expected to require as a minimum roughly an additional 20% printing energy to deliver the required printing density. This is a serious disadvantage since the life of the costly printhead is a strong inverse function of the power expended in printing.
Fluorinated compounds have been disclosed for use in anti-stick layers and release layers. For example, EPO Publn. No. 263,478 discloses thermal transfer imaging materials with an anti-stick back layer. The transfer layer on the front surface comprises a non-flowable ink layer and an adhesive layer. An anti-stick back layer has a thickness in the range of 0.05 .mu.m to 3 .mu.m and contains a main component chosen from fluorine-containing surface active agents and fluorine-containing polymers. These main components are preferably mixed with heat resistant resins such as epoxy resins, silicone resins, phenolic resins, melamine resins, and polyester sulfones, and others. The fluorine containing polymers disclosed are tetrafluoroethylene-hexafluoropropylene copolymers, polychlorotrifluoroethylene, polyvinylidene fluoride, and polytetrafluoroethylene.
Japanese Kokai JP63-062790 discloses the use of a cellulose derivative mixed with a resin which can be fluorinated. No further definition of the latter resin is provided, however, and such compounds are not disclosed in the examples. The layer thickness is stated to be in the range 0.05 .mu.m to 3 .mu.m.
Japanese Kokai JP63-074687 discloses anti-stick back layers of thicknesses in the range 0.2 .mu.m to 5 .mu.m containing polyurethane fluoride as the major component.
Japanese Kokai JP63-118296 discloses thermal transfer materials with the heat transfer layer on one side of a support. On the other side of the support is a heat resistant layer which contains a perfluoroalkyl group-containing resin preferably selected from oligomers of tetrafluoroethylene and hexafluoropropylene, or from copolymers of a perfluoroalkyl group-containing vinyl monomer with (meth)acrylic acid or esters.
U.S. Pat. No. 4,631,232 discloses a heat-sensitive transferring recording material having a heat melting ink layer on one side of a substrate, and a "heat resistant conveyance improving layer" on the other side of the substrate. The conveyance improving layer comprises either UV radiation curable resins or compounds containing a perfluoroalkyl group. The former are illustrated by polyester acrylate, polyurethane acrylate, and epoxy acrylate, but no indication is made that the monomers in the curable resins may be fluorinated. The compounds containing perfluoroalkyl groups are not described as polymerizable monomers and are not UV cured; they are represented by salts or esters of perfluoroalkyl carboxylic acids, salts of perfluoroalkyl sulfonic acid, esters of perfluoroalkyl phosphoric acid, perfluoroalkyl betaine, and perfluoroalkyl trimethyl ammonium salts.
U.S. Pat. No. 4,829,050 discloses thermal transfer materials with an anti-stick back layer comprising Teflon.TM. particles dispersed in a cellulose binder.
U.S. Pat. No. 4,383,878 discloses a transfer material for indicia wherein a first support base to which the indicia are applied has a release topcoat comprising a radiation-curable polyfluorinated acrylate compound and a polyethylenically unsaturated crosslinking agent.
U.S. Pat. No. 4,321,404 discloses the same radiation-curable compositions disclosed in U.S. Pat. No. 4,383,878 and presents utilities involving the controlled release of images applied to the adherent composition layers.
Although the foregoing disclosed anti-stick and release coatings containing fluorinated compounds are suitable for their intended use, improvements in such coatings are continually sought and desired by the industry for use in dye thermal transfer systems. Specifically, improvements with respect to the heat resistance, lubricity, dye impermeability, and self-cleaning properties of anti-stick coatings for thermal dye transfer sheets are constantly needed.